Metal Adsorption Controls Stability of Layered Manganese Oxides

Hexagonal birnessite, a typical layered Mn oxide (LMO), can adsorb and oxidize Mn(II) and thereby transform to Mn(III)-rich hexagonal birnessite, triclinic birnessite, or tunneled Mn oxides (TMOs), remarkably changing the environmental behavior of Mn oxides. We have determined the effects of coexisting cations on the transformation by incubating Mn(II)-bearing δ-MnO₂ at pH 8 under anoxic conditions for 25 d (dissolved Mn < 11 μM). In the Li⁺, Na⁺, and K⁺ chloride solutions, the Mn(II)-bearing δ-MnO₂ first transforms to Mn(III)-rich δ-MnO₂ or triclinic birnessite (T-bir) due to the Mn(II)-Mn(IV) comproportionation, most of which eventually transform to a 4 × 4 TMO. In contrast, Mn(III)-rich δ-MnO₂ and T-bir form and persist in the Mg²⁺ and Ca²⁺ chloride solutions. However, in the presence of surface adsorbed Cu(II), Mn(II)-bearing δ-MnO₂ turns into Mn(III)-rich δ-MnO₂ without forming T-bir or TMOs. The stabilizing power of the cations on the δ-MnO₂ structure positively correlates with their binding strength to δ-MnO₂ (Li⁺, Na⁺, and K⁺ < Mg²⁺ and Ca²⁺ < Cu(II)). Since metal adsorption decreases the surface energy of minerals, our finding suggests that the surface energy largely controls the thermodynamic stability of LMOs. Our study indicates that the adsorption of divalent metal cations, particularly transition metals, can be an important cause of the high abundance of LMOs, rather than the more stable TMO phases, in the environment.